Fluorescent lamp starter



June- 26, 1945. c. H. HODGKENS FIsUOESCENT LAMP STARTER 2 Sheets-Sheet 1Filed Feb. {1, 1942 INVENTOR '6'. HODGK/AS. BY 10%,, l .k 5

' ATTORNEY June 26, 1945. c. H. HODGKINS FLUORESCENT LAMP STARTER FiledFeb. 4, 1942 2 Sheets-Sheet 2 INVENTOR c'. H. HODGIfi/VS ATTORNEY tion.

" Patented June 26,

, Charles n Hodgkins, Fail-seal, cums,

Company, Conn., a corporation of Connecticut Application February 4,1942, Serial No. 429,461

The Bryant Electric assignor to I Bridgeport,

, 11 Claims. (Cl. 2oo-122 I The present invention relates to thermalrelays;

and more particularly to such relays for thestarting of electricdischargelamps of the fluorescent ype Lamps of the fluorescent type arenow wellknown to the art and in order to operate such lamps from thecustomary domestic source of potential of about 115 volts, they areprovided with filamentary electrodes which must first be heated to anelectron-emitting temperature prior to application of a sufiicientlyhigh starting voltage across the preheated electrodes.

To obtain this sequential operation, a relay is customarily employedwhich first connects the filamentary electrodes of the lamp in a seriesheating circuit to the source, and after the electrodeshave been heatedto-electron-emitting temvention to provide a relay which may be em-vperature, the relay opens this series heating cir-,

cuit causing a high voltage surge from an inductance element necessarilydisposed between the I source and one'of the electrodes of the lamp.

This high voltage. surge is usually sufliciently above line H voltage tocause a discharge between the preheatedlamp electrodes, and oncestarted, the operating voltage of the lamp is below line voltage so thatthe discharge is readily sustained at the availabl'domestic potential of115 volts. r

If for any reason the discharge isj not initiated on the first operationof the relay, it continues to perform its cycle of operation until thedischarge is actually struck-between the electrodes and the lamp finallystarted. Thus there are several primary requisites for a starting relayfor this purpose. .It must first maintain the series heating circuitclosed for a period of time sumcient ,to bring the-filamentary lampelectrodes up. to full electronsemittingtemperature prior tointerrupting this circuit with causation of the high voltage surge fromthe inductance element,'otherwise. the

filamentary .electrods will be stripped of their electron-emittingcoating 'by the high voltage surge. Secondly, the relay must, as abovenoted, continue its cyclic operation provided the lamp start the lamp Ito prevent the consumer from waiting in thedark for the lamp to giveillumina- Relays of this type have heretofore had disadvantagesresulting from their above noted requisites. One such disadvantage hasbeen that a relay of diife'rent electrical characteristics has beenrequired for practically each wattage fluorescent lamp because of thediiference in size 01; of Fig. =1, showing position at the instant offailsto start the first time, and its entire cycle of operation mustoccur as quickly as possible to rangement' for starting and operating'afluoresso centlamp and showing-the; normal position of' the filamentaryelectrodes for the various wattage lamps and the voltage that musttherefore be applied. In addition, the primary requisite for repetitiousoperation of the starting relay is itself a distinct'disa'dvantage undercertain conditions.

For example, in the case of a failed fluorescent lamp the relay hasheretofore continued its cycle of operation in a vain attempt to startthe lamp in the same manner as though the lamp had merely failed. tostart on the first few operations of the relay. This not onlygreatlyaffects the useful life of the relay itself, but causes flickering of.the failed lamp, which is a source of considerable a-nnoyanceto theuser.

It is accordingly an object of thepresent in ployed for startingfluorescent lamps regardless of their particular wattage.

Another object of the present invention is the provision of a relay forthe starting of fluorescent discharge lamps which automatically operatesto effectively disconnect the lamp from the source whenever the lampfails for any reason.

- Another object of the present invention is the provision of a relayfor starting fluorescent dis-1 will become obvious reference to theaccompanying. drawings wherein:

Fig.1 is a front elevationalview of the prebut taken at to the latter;

I 1m 3 is a fragmentary sectional view taken in I theIineIH- -IIIo fFIgLI;

charge lamps which may be universally employed for starting such lampsregardless of their par- 7 ticular wattage and wherein the 'relay is.au'wmatically operable "to disconnect thelamp from the source of supplyin the event the lamp has I failed. I w I Another'object ofthe presentinvention is the provision of a thermal relay for starting fluorescentdischarge lamps which performs its sequentialoperation with fidelity andical to manufacture;

still further objects of the present invention to thosefskilled in theart" by ferred form of starting relay for fluorescent lamps and showingthe container therefor in section;

Fig. 2 is a fragmentary view similar to Fig. 4 is a schematic diagram ofa circuit at}- the relay of the present invention before, or a second orso after, closure of the lamp energizing whichis econ'omrelay whenemployed with lamps of a wattage in excess of 30 watts;

Fig. 8 is a sectional view VIII-VIII of Fig. 1:

F1893 is an elevational .view

r the bimetallic element of the relay of Pig. 1;

( Fig. 10 is an elevational view of a modification which the startingrelay of the present invention may take;

Fig. 1'1 is a side elevatlonal view of the modification shown in Fig.10, and v v,

Fig. 12 isa sectional view taken on. the line xix-4m of mg. 10. ,i o

Referring now to the drawings in detail. the starting relay "of thepresent invention is shown takenon the linegenerallyatIintheseveralilguresandisho sc fv within a metallic orplastic casing I together with a radio frequency condenser I, both ofwhich are connected in electrical parallel to a pair of contactterminals U and I adapted to engage a socket,

andsecuredtoaninsulatingclosuremember ll.-

The relay 117881! a base HOf suitable insulating material such as fiber,a phenolic condensation product, hard rubber, or the'iike,

and isv provided with a rectangular open '3 therein. A substantiallyU-shaped heat-responsive element II in the form of a bimetallic memher,having one leg thereof longer than the other,

issecuredalongthelongestl'egtoaneyelet or the like ll, which forms apivoted support and,

fllamentron-emitting temperature prior to the initiation of a dischargebetween these electrodes. To this end one side of the electrode fl ismnnected by a conductor ll, inductance l2, and switch 88, to

one side of the domestic source of supply of .ap-

proximately 115 volts, and one side of the electrode 28 is likewiseconnected by a conductor 34 to the other side of this source. In orderto complate a series heating circuit for these electrodes 28 and 2|, theremaining ends of eacheare connected together by the relay I which issecured to a suitable socket (not shown) so that the terminals I and 8together with conductors II and Il complete the circuit.

Upon closure of the switch 33 a series heating circuit will be thuscompleted through inductance II, electrode ll, relay 5, and electrode itback to the source LI-LI, which may be a transformer or a domesticsource of ll5-volts, as is well-known. in the art. Since this circuitarrangement per se is well-known, it is believed unnecessary to tracethe circuit in its entirety, and consequently further description willbe confined to the relay "8. The current passing through the relay 5with the switch 83 closed wlil iiow from the contact terminal I andstrap II to .the eyelet 20. Since one end of the resistance heater II isconnected to this eyelet 20 by the conductor 12, current flows throughthis heater to heat collector it, .through normally closed contact II,the upper portion of the longest leg of heat-responsive element 13, andt ence through supporting eyelet I l and strap I to contact terminal I,thus subjecting the heater I! to full line voltage. 7 I

Inasmuch as the heater II is disposed longitudinally of the longest legof the heat-responsive element It between its pivot I4 and the base ofthe U, thehea't generated by the application of the voltage from thesource, which will be across electrical connection for theheat-responsive ele-.-

ment II, with this eyelet'il being connected to theoontactterminallbyametallicstrap II and to one sideof the condenser 1 by a conductor It.

The free'end with a suitable contact I! normally engaging a metallicheat collector It in the form of a supporting strap having its mgularlydisposed end i (Fig. 2) rigidly'secured to the base" H. A high ohmicresistance heating element I! is electrically and mechanically securedto the heat collector II and disposed longitudinally in juxtaposition'to the longest leg of the heat responslvc element II between its supportIt and the bottom of the U, with its other end electrically connected toan eyelet II by means of a conductor 22. This eyelet fl, like the eyeletll, passes through'the base of therelay and is connected by a strap:orthelikelltothemmttemhialllhdw v of the longest leg of the U- shapedheat-responsive element II is provided iieetion of that portion,together with thebottomtheheater, hereinafter referred to as full linevoltage, is rapidly transferred to the adjacent portion of theheat-responsive element ll. De-

of the U, occurs, in the direction of the base H with the bottom of theU passing inwardly of the rectangular slot I! provided forthat purpose.Deflection in the direction of the base Ii will socordingly cause theshortest leg of the heat-retheremainingsideoftheconden-serlhyccon--(motor as (no. a contact as is electrically.andmechanicallysecuredbytheeyeletllsoas to benormally i wedinspacedrelationtoa contact Ilrigidlysecured totheshortest leg of theheat-responsive element It so that these two contacts will engage eachother as hereinafter Referring now'more particularly to Fig.1, the

operationoftherelaylwillbedeseribedincon neetlon with itsintendedpurpose of starting a fluorescent lamp l'l. the'lampll is th filamentaryelectrodcsfiandllwhiehnmstbeheatedtoelecvAsrshown-in this fi ure;

Bio:- anyreasonthe dischargoisnotinitiated' sponsive element to moveaway from the base until contact II engages contact 2!, in the mannotshown in Fig. 5.

This enga ement of the contacts 2 and II occurs very rapidly, requiringonly a very few seconds,. with the result that a low resistance heatingcircuit is provided for the lamp electrodes fl and 20, allowing them tobe heated quickly to maximum electron-emitting temperature, since .theheater II is short-circuited by the engaget-circuitment of the contactsII and I. Shot ing of the resistance heater It results in no furthergeneration of heat so that the heat-responsive element ll coolssuillciently to'cause it to return to its normal position with attendantseparation of the contacts It and 2|. Separation of the latteraccordingly causes a high 'voltsge surge from the inductance element 82which is impressed across thepreheamd lamp electrode Ilyand ",thusstriking the are with then being in theposition as shown in I'll.

1m the'flrst operation offthe relay. I, thegheatsr Is isliainimmediately hibiscus tofullline voltage, withthe result that itwill repeat it cycleof operation a number of times until the dischargeis initiated.

Once the lamp -2'I is started, the heater. I9 is then subjected only to-lamp voltage, and on lower wattage lamps the size of the heater "andarea or the heat-responsive element ll are so designed thatinsui'iieient heat is generated at lamp voltage to materially aflect theheat-responsive element [3 or to cause closure of the contacts is and aduring operation of the lamp. 10

For higher wattage lamps which necessarily require a higher lampvoltage, suflioient heat-would unavoidably be generated to cause closureof thecontacts 34 and It in the absence of provision ,to the contrary.Accordingly; in order to universally adapt the present relay .to allcommercial wrtisage lamps, the. heat collector ll and contact I areprovided. I

In the case of higher wattage lamps where the heater is subiect to avoltage approximating line voltage during operation oi the lamp, theheat generated bythe heater I! will be carried to the heat collector II;and since the latter is disposed adjacent the upper portion ofthe-longest, leg oi the U-shaped'heat-responsive element It which is'inthe electrical'ci'rcult, this portion will be and into the rectangularopening 42. Again, this deflected by the heat toward the base u, thusopening the contacts I! and I8 before the contacts 2 5 and It can close,as shown by the" position of the relay in Fig. 'l. Inasmuch as this completely disconnects the heater I9 from the source,

there will be a continual opening and closing of the contacts I1 and I!as the heat-responsive eiducting material, such as silver or copper, andis of such-shape and proportion that 'it';will permit a number 01operations of. the relay 5 before thermally "locking" the relay in anopen circuit condition, as well as permitting closure of the contacts 25and 26, in the case of higher wattage lamps before causing continualopening and closing of the contacts l1 and 18. Moreover, the

thermal mass of the heat collector l8 should -be' heat will again betransmitted to'the U-shaped I heat-responsive element 43 with attendantdeflection of the leg adjacent the heater, and the bottom of the U inthe direction of tire base 4| will cause movement of the other legor'the U away from the base 41 until the contact 46 carried by the freeend thereof engages the stationary contact 45 carried by the eyelet 40,as previously described relativeto the preferred modification of Fig. 1.The heater 49 isv thus shortement responds toheating and cooling, due tointermittent current flow through the heaterjll.

However,- in the case of allwattage lamps, should it fail to start onthe first separation of contacts 25 and, the heater II will again be,subject tofull line voltage with repetitious operation of the relay asabove noted. I

-Inaddition to the relay 0! the present invention being universallyadapted for all commercial wattage-lamps, it i also operative to"lock-out the discharge lamp should it fail, due to deactivation of theelectrodes oBsimilar cause. In an instance of this kind the relay 9 willperiormitsl cycle of operation a number oi times corresponding to apredetermined period or time, as before -mentloned, and during eachattempt to start the lamp 21, the resistanceheater is will be subjectedto full line (as distinguished from a high lamp voltage as'in of highwattage lamps) with the heat generatedthereby being accumulative. Thetemperature of heat collector l! accordingly will finally reach a valuethat will;

and making of the heat; ing circuit by opening and closing or thecontacts cause a cyclic breaking Fl and I9. Durin time is insuiiicientto permitheating of the lower portion of the U-slvapedbimetallic-element I! to Consequently. attempted starts of the lamp willbe eliminated since it will be "locked-out" due to the inability oi thecontacts 2| and 16 to close again, with the relay I remaining in theposition the cyclic operation, the closedclrcuit '{shown in Fig. 7. Thecurrent flowing through the ballast or inductance 32 will be reduced tothe circuited and upon cooling of the heat-responsive element 43, thecontacts 45 and-46 separate to causethe high voltage surge withinitiation of the discharge between the lamp electrodes.

After. starting of the lamp, the heater 49 will again be subjected only'to lamp voltage at which insufficient heat. is generated by heater 49to cause closure of 'contac 45 and. Should the lamp fail to start onthegflrst operation of the relay 5, it willcontinue its cyclic operationseveral times until the discharge is finally initiated in the H lamp. Inthe event, however, that the lamp has failed due to deactivation of theelectrodes'or similar cause, the heater 49 will be subjected to fullline voltage each time the contact separate. As in the previousembodiment, a heat c0lleo-' 0- tor 49 is provided whichin themodification of Figs. 10, 11, and 12 partially surrounds'and intimatelycontacts the heater 49 and is so shaped and arranged tratanother portionI0 is disposed in juxtaposition tothe' leg of the U carrying thecontact-48. The heat generated by repeatedly subjecting theheater 49 tofull line voltage is again accumulatively transmitted to the heat Icollector 48 which accordingly flows to the portion 99, causing the legof the u-shaped bimetallic element carrying the contact 46't0 be heatedand to deflect toward the base 4|, thus preventing e closure of thecontacts 49 and 46 and "lockingt e point where. contacts I! and 29 willclose.

value which flows through the heater l9 dur--' ing the closedv portionor the cycling of the heater circuit, thus preventing detrimental effectto the inductance, as well as to the relay I itseli by *overheatingjor aprolonged; period or time.

'To accomplish the operation as above noted, the heat collector II mustbe'oi' good heat conout the defective lamp, thereby eliminatingattempted starts with annoying lamp flicker. The

current in the ballast 32 is also again reduced to that value whichflows "through high ohmic rer-istanoe heater 49. thus protecting theballast agrinst detrimental or destructive current. Upon replacement ofthe failed lamp, the relay 5 again returns to its normal position readyfor its cyclic operation.

It thus becomes obviou lamps of the fluorescent type is herein providedwhich is simple and economical to manufacture.

,Moreover, such relayis universally operable to to those skilled m' theart that a thermal relay for starting discharge start fluorescent lam sor all commercial watt ages and in addition "looks out a railed lamp soas to eliminate annoying flicker oi the lamp,

and needless repetitious operation of the relay.-."

Upon replacement oi the failed lamp, however, I the relay automaticallyreturns to its normal position and operation. r 7

Although severallembodiments oi the present invention have been shownand'descrlbed, it is to-be understood that still iurther modificationsmay be made without departing from the spirit and scope of the appendedclaims. 1

I claim:

l. A thermal relay ior controlling an electric circuit comprising abase, heat-responsive means carried by said base, an electric heaterdisposed in nest transfer relation tosaid heat-responsive means, meansoperable in response to heating of said heat-responsive meansby saidheater upon the application or approximatelyline voltag to the latter toclose the circuit and short-circuit i said heater, and means in heattransfer relation with said heater and said heat-responsive means foraccumulating heat therein upon the application approximately linevoltage to said heater for a predetermined time and operable to preventsaid heat-responsive means from causing circuit closing operation oisaid flat-mentioned means and to cause said heat-responsive meanstoretain said flrst-mentioned means in arropen cirso proximately linevoltage to the latter to close the circuit and short-circuitsaid'heater, circuit-opening means associated with said heat-responsivemeans and normally included in the circuit, and means in 'heat transferrelation with saldheater .and said heat-responsive means !oraccumulating heat therein upon the application oi approximatelyglinevoltage to said heater ror a prede- \termined time and operable to causesaid heatresponsive means to operate said circuit-opening means withattendant interruption oiithe circuit thus preventing saidheat-responsive means from 5 operating said circuit'closing means withshortcircuiting of said heater. v

3. A thermal relay for starting fluorescent a lamps of various wattagesfrom secures of electricalenergy comprising a base, a heat-responsiveelement secured to said base, an electric heater disposed adjacent saidheat-responsive element for transmitting heat to the latter upon theflow of current through said heatencircuit-cloaing of saidheat-responsive element by said heater upon the application or full linevoltage to said lamp and to said heater, to close the circuit to said.lamp and short-circuit said heater and operable toopen upon starting ofsaid lamp and- 65 cooling ofsaid heat-responsive element; andcircult-opening means associated with said heat-responsive element 'andnormally included inthe circuit with said lamp and said heater, andoperable to cyclically open andclose said circuit upon '70 continuedheatins or said heat-responsive element,

by the application oi lamp voltage to said heater 1 when such voltageapproximates line voltage with higher wattage lamps, to said circuit andprevent said heat-responsive element from- 7 10 means until said lamp ismeans in heat transfer relation with said heater operating saidcircuit-closing means and shortcircuiting said heater.

.4. A thermal relay ior starting fluorescent lamps of various wattagesfrom a source or electrical energy comprising a base, a heat-responsiveelement secured to said base, an electric heater disposed adiacent saidheat-responsive element for transmitting heat to the latter upon theflow or current through said heater, circuit-closing normally operablein responseto healing or said heat-responsive element by said heaterupon the application or full line voltage to said lamp and to saidheater, to close the circuit to said lamp and short-circuit said heaterand operis able to open upon starting or said lamp and cooling oi saidheat-responsive element: circuitopening means associated with saidheat-responsive element and normally included in the circuit with saidheater, and means in heat transier relation with said heater and saidheat-responsive element ioraccumulating heat therein upon con--application of lamp voltage to said heater when such voltageapproximates line voltage with higher. wattage lamps and operable tocause said said heat-responsive element to operate said circuit-openingmeans with attendant interruption of the circuit to prevent saidheat-responsive element irom operating said circuit-closing means withshortrcircuiting of said heater.

5. A thermal relay for starting a fluorescent lamp from a source ofcommercial voltage comprising a base, a heat responsive element securedto said base, an electric heater disposed adjacent said heat-responsiveelement for generating sum-- as cient heat only upon the application orfull line voltage thereto to cause deflection oi said heatresponsiveelement and incapable of -generating suflicient heat to materiallyafl'ect said heat-responsive element at a voltage below approximatelyfull line voltage, a pair oi normally open contacts operable, to engageeach other and short-circuit said heater upon the initial heating orsaid heat-responsive element by said heater in response to theapplication of full line voltage thereto, and heat-collecting means inheat transfer relation with said heater for accumulating heat thereinupon repeated line voltage to said heater for a predetermined period ortime and operable to render said heatresponsivs element inoperative toclose and opersaid lamp for generating suflicient heat only upon meansnormally operable in response to heating 00 element, a pair 01' normallyopen contacts cyclically operable to engage each other and short-circuitsaid heater upon the initial heating oi said heat-' responsive elementby said heater in response to the application of full line voltagethereto and to open upon cooling 0! said heat-responsive elementstarted, -arid heats-collecting for accumulating heat thereinuponrepeated application oi full line voltage to said heater ior apredetermined period or time and operable to to close and operable tomaintain said contacts in an open Position application or full 3 asrasoo'1. A thermal relay for starting a fluorescent lamp from a'source ofcommercial voltage comprising a base, a heat-responsive element securedto said base, an electric heater disposed adjacent said heat-responsiveelement for generating sufllcient heat upon the application or full linevoltage thereto to cause deflection of aid heat-responsive element, apair of normally open contacts operable to engage each other andshort-circuit said heater upon the application of normal line voltage tosaid heater and initial heating of said heat-responsive element, a pairof closed contacts for normally connecting said heater to said source,and means operable upon the application of line voltage to said heaterbeyond a predetermined period of time to cause said pair or closedcontacts to open and disconnect said heater from said source to preventsaid heater from operating said heat-responsive element with attendantclosure of said normally open contacts. I

8. A thermal relay for starting a fluorescent lamp from a source ofcommercial voltage com- Drl lng a base, a heat-responsive elementsecured to said base, an electric heater disposed adjacent saidheat-responsive element for generating sutflcient heat only upon theapplication of approximately full line voltage thereto to causedeflection of said heat-responsive element, a pair of normally opencontacts operable to engage each other and short-circuit said heaterupon the initiai heating of said heat-responsive element by said heaterin response to the application approximately lull line voltage thereto,a pair of closed contacts -lor ndrmally connecting said heater to saidsource, and heat-collecting'means operable upon continued application ofapproximately line voltage to said heater beyond a predetermined periodof time to heat said heat-responsive element and cause said normallyclosed contacts to open and disconnect said heater from said source toprevent said heater irom operating said heat-responsive element withattendant closure of said normally open contacts.

9. A thermal relay for starting a fluorescent lamp comprising a base, abimetallic element carried by said base, an electric heater disposed injuxtaposition to a portion or said bimetallic ele- .ment and operableupon the initial application of approximately full line voltage theretoto cause deflection oi the adjacent portion of said bimetallic elementin one direction, and the remaining portion thereof in anotherdirection, a contact carried by one end of said bimetallic element andengageable by said base for closing a circuit to said lamp and forshort-circuiting said heater, and a heat collector in heat transferrelation with said heater and the portion or said bimetallic elementremote from saidlieater for accumulating heat therein upon continuedapplication of approximately full line voltage to said heater andoperable to cause deflection of the portion of said bimetallic elementremote from said heater in the same direction as the portion adjacentthereto to prevent engagement oi said contacts with attendant closure ofsaid circuit and shortcirculting or said heater.

10. A thermal relay for starting a fluorescent lamp comprising a base, aU-shaped bimetallic element carried bysaidbase, an electric heaterdisposed in juxtaposition to one leg of said U- 5 shaped bimetallicelement and operable upon the initial application of approximately fullline voltage thereto to cause deflection of the adjacent portion of saidbimetallic element in the direction of said base with attendantdeflection of the leg thereof remote from said heater away from saidbase, a contact carriediby the endof the leg of said U-shaped bimetallicelement remote from said heater and engageable with a stationary contactterminal carried by said base for closing [5 a circuit to said lamp andfor short-circuiting said heater, and a heat collector in heat transferrelation with said heater and the leg of said U-shaped bimetallicelement remote from said heater and accumulating heat therein uponcontinued application of iull line voltage to said heater and operableto cause deflection or the leg of said U-shaped bimetallic elementremote from said heater in the same direction as the portion adjacentthereto to prevent engagement of said contacts with attendant closure ofsaid circuit and 'sho'rt-circuiting of said heater.

11. A thermal relay for startinga fluorescent lamp comprising a base, aU-shaped bimetallic element carried by said base, an electric heaterdisposed in juxtaposition to one leg of said U- shaped bimetallicelement and operable upon the application or approximately full linevoltage thereto to cause deflection or the portion of said bimetallicelement in the directionof said base with attendant deflection or theleg thereof remote from said heater .away from said base, a spacedcontact carried by the .end of the leg of said U'-shaped bimetallicelement remote from said heater and engageable with a stationary 4contact terminal carried by said base upon deflection of the leg or saidU-shaped bimetallic element for closing a circuit to "said lamp and forshort-circuiting said heater, a contact carried by the end of the leg ofsaid U-shaped bimetallic "element adjacent said heater and normallyengaging a stationary contact carried .by said base,

7 and a heat collector in heat transfer relation with said heater andtheportion of said U-shaped bimetallic element carrying the normally closeda contact for accumulating heat therein upon continued application ofapproximately full line voltage to said heater and operable to causedeflection of that portion of said U-shaped bimetallic element adjacentsaid heat collector to cause disengagement of the normally closedcontact carried thereby from the stationary contact carried by said baseto interrupt the circuit to said heater to prevent the latter fromheating said.

U-shaped bimetallic element to a temperature suflicient to causeengagement of the spaced contact carried by the leg thereof remote fromsaid heater with the adjacent stationary contact carried by said basewith attendant closure-of said 6? circuit and short-circuiting of saidheater.

CHARLES H. HODGKINS.

